The present invention relates generally to electrical lamp fixtures used for general-purpose lighting, and specifically to an improved light emitting diode (LED) illumination apparatus, incorporating an LED array in which multiple phosphors have been dispensed onto the LEDs, in order to improve the color rendering, color mixing, and color temperature control of the apparatus.
In order for LED illuminators and light engines to act as a satisfactory replacement for traditional general-purpose lighting, it is desirable and even necessary to produce white light with characteristics that are similar to the light produced from an incandescent bulb, or in some case, to accurately replicate the light provided by natural sunlight. This is especially important for lighting applications that demand high quality light with well-controlled parameters, such as lighting for professional photography, videography, and the motion picture industry. In a general sense, this means that the LED illuminator or light engine should have a spectral response or characteristic that mimics the spectral response of an incandescent bulb, and/or natural sunlight, at specific color temperatures. The spectral characteristics of both the LED illuminator and the target light sources can be expressed in the form of a spectral plot of light emission as a function of wavelength, and also in terms of related measures including Correlated Color Temperature (CCT), hue (which can be quantified using CIE chromaticity diagram coordinates), and Color Rendering Index (CRI).
Briefly, the Correlated Color Temperature (CCT) of an illuminator or lamp is the color temperature of a black body radiator which to human color perception most closely matches the light from the lamp, and is typically expressed in degrees Kelvin (K). In practice it is primarily applicable to white light sources. A typical incandescent bulb will have a CCT in the range of 2500-3000K, typically referred to as “warm white”. Illuminators with higher CCT values may be described as “cool white”. Light from the sun may have CCT values in the 5000-6500K range, depending on time of day, the height of the sun above the horizon, and also the degree of overcast. It is a highly desirable attribute for an LED illuminator to have a well-defined and controlled color temperature, with CCT values ranging from approximately 2500K to 6500K or even higher, depending on the application. LED illuminators may also provide variable color temperature, either through a finite number of CCT settings, or via continuously varying control.
Color Rendering Index (CRI) provides a quantitative measure of a light source's ability to faithfully reproduce the colors of illuminated objects, in comparison to an ideal or natural light source. For the comparison to be valid, the test light source and the reference source must be of the same color temperature. For light sources above 5000K, daylight is used as the reference source. For light sources under 5000K, an ideal black body radiator of the same color temperature is used. A full description of the measurement of CRI is beyond the scope of this document. However, the basic measurement process consists of measuring the light reflected from a series of test color samples, when illuminated by the test light source and the reference light source. In practice, software packages that are provided with commercially available visible light spectrometers are able to compute the Color Rendering Index of light sources. In principle, natural sunlight will have a CRI of 100, and the light emitted by an ideal black body radiator will also have a CRI of 100.
It is also possible to specify or quantify the hue or color of light using CIE chromaticity diagram coordinates. The color coordinates of an ideal black body radiator, taken at different CCT values, are represented on a CIE chromaticity diagram as a curved line segment. However, it is important to note that the color coordinates of a light source do not provide any indication of the CRI of the light source.
LED illuminators that are intended to produce white light for general illumination purposes, face two significant challenges. They should provide light of the intended color temperature, generally in the range of 2500K to 6500K, depending on the desired appearance and application, with CIE chromaticity diagram color coordinates that lie on, or very close to, the black body radiation curve. What is meant by “very close to” will be explained below. They should also achieve a high CRI, as close to 100 as possible. This is especially important for demanding applications such as in the fields of professional photography, videography, and motion picture filming. By using a mix of red, green, and blue LEDs, it is easy to provide any desired color temperature. However, the color rendering of such an RGB LED illuminator will be very poor, with CRI values in the 70's, or even lower. This is due to the fact that the RGB LEDs have narrow bandwidth emission, with FWHM (full width at half maximum) bandwidths of only 25-30 nm, for each of the three LED colors/types. For example, test objects that reflect significant amounts of yellow wavelengths will not render accurately, due to an RGB LED illuminator's lack of emitted light in the yellow region. Even if amber LED chips are added, there is a “dead zone” that is roughly in the range of 550 to 590 nm in which LEDs have very low emission efficiency, making it extremely difficult to obtain CRI values above 92%, even when using a large number of LED wavelengths.
The most common method for obtaining good color rendering from an LED illuminator is to coat blue LEDs with phosphors that absorb light energy from the blue LEDs, and convert a portion of this energy into broad-spectrum emission at higher wavelengths, typically with a spectral peak in the yellow region of the visible light spectrum. In this document, higher wavelengths and longer wavelengths have identical meaning and are used interchangeably. Typical phosphors have FWHM bandwidths of approximately 50 to 120 nm, and therefore provide greater spectral fill than individual LEDs. This approach can provide reasonably good color rendering, with a fixed color temperature. However, it can be difficult to accurately control the color temperature that results, and it may also be difficult to achieve lower color temperatures, such as “warm white”. For this reason, some prior art embodiments add red LEDs, as a means of “warming” the light output, and also potentially offering the ability to vary the color temperature of the illuminator. While the addition of red LED chips provides advantages in terms of color temperature control, the narrow spectral bandwidth of the added red LEDs has limited benefit in terms of color rendering, and in fact may actually reduce the CRI of the light output as the output of the red LEDs is increased. The CRI of such an illuminator is determined primarily by the spectral characteristics of the phosphor that is used to coat the blue LEDs.
Due to the limitations described above, there exists a need for an LED illuminator that provides the combined light output from a cluster or array of multiple LED chips, for applications that demand high-quality lighting. In addition to providing the usual advantages of LED lighting, in terms of energy efficiency, long life, and reliability, it desirably provides a well-controlled Correlated Color Temperature (CCT), preferably with the ability to vary the CCT over a wide range via some form of user control. It preferably also provides extremely good color rendering, with CRI values that exceed 95, and ideally achieve CRI values of 98 and above, throughout the illuminator's full range of CCT settings. Finally, the light from the LED illuminator is preferably highly uniform, in terms of color and hue, over its field of view.